It is known from U.S. Pat. No. 3,629,435 that the following compounds possess analgesic activity: ##STR3## wherein R is benzyl or substituted benzyl. In this patent R is broadly defined but it appears that only a few substituted benzyl derivatives were actually prepared, specifically, only the ortho-chlorobenzyl derivatives among which 1-[1'(o-chlorobenzyl)-2'-pyrryl]-2-di-sec.-butylaminoethanol is named: ##STR4## This compound is said to be the most active compound within the disclosed class of aminoethanols and it has been marketed as a valuable analgesic under the name "VIMINOLO" (W.H.O. Chronicle, 1970, N.3, list 25).
According to U.S. Pat. No. 3,629,435, the compounds II and III are prepared as a mixture of stereoisomers by reducing the corresponding keto precursors. Despite many attempts to resolve the said stereoisomeric mixture, which can be more or less complex because of the number of assymetric centers present in the molecule, it has been impossible up to now to do so.
The U.S. Pat. No. 3,857,857 describes a method for directly synthetizing each optical isomer of 1-[1'(o-chlorobenzyl)-2'-pyrryl]-2-di-sec.-butylaminoethanol (III) and using this method four isomers of optical purity were prepared and their analgesic activity determined.
From this determination it was found that one of the four isomers, identified in said Patent as Compound "R.sub.2 ", is much more active than the other isomers and also, more active than the corresponding racemic compound "VIMINOLO".